The WebReference.com website has an introductory tutorial showing the budding PHP develper how to get started with one of the keys to web application interaction - forms.

Forms are how your users talk to your scripts. To get the most out of PHP, you must master forms. The first thing you need to understand is that although PHP makes it easy to access form data, you must be careful of how you work with the data.

The first part of the tutorial is focused on something many applications don't worry about - the security and integrity of the data submitted to them. They talk about things like filtering and various other checks to ensure that the data you're getting is good. They also mention the method for putting submitted values into PHP arrays, checking for valid values, using multiple submit buttons and an example of some of these methods all put together - validating a credit card number.